1. Field of the Invention
The present invention relates primarily to the field of water treatment UV systems, and in particular to a dielectric barrier discharge lamp used in fluid treatment UV systems, where the irradiated fluid is used as a low voltage outer electrode.
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2. Background Art
UV irradiating of water is a viable alternative to not only chlorinating water (especially drinking water) to disinfect it of harmful bacteria, but also for the degradation of organic compounds in fluids by advanced oxidation processes. UV irradiation is a mature alternative to chlorinating water because it eliminates the use of chemicals that may be allergic or harmful to the user. While disinfection of water can be primarily achieved by stand alone UV irradiation using UV light, advanced oxidation systems typically need a combination of UV light and oxidizing agents (ozone and/or hydrogen-peroxide) to reduce organic compounds found in certain fluids. Some advanced oxidation systems also include dispersed photo-catalysts, such as Iron (Fe) or Titanium (Ti) in addition to oxidation agents and UV light.
Using any of the above mentioned processes, the UV-treatment system must be able to effectively irradiate the fluid with (V)UV radiation since the (V)UV photons start the desired photo-physical or photo-chemical reaction.
Reactor Vessel
A cross-section of a typical reactor vessel with an imparted UV source used in the disinfection of water is shown in FIG. 1. Reactor vessel 101 is made out of metal, quartz, or glass, and forms the outer capsule. Infected water 102 enters the vessel via water inlet 103, and the disinfected water leaves the vessel via water outlet 107. The reactor vessel houses the UV source 106 which is encapsulated inside an inner sleeve 100 made out of quartz, or fused silica. The reactor vessel and sleeve are joined together at appropriate places using O-ring seals 105. The UV source is powered using power supply 104.
Currently such a system is widely used, for example, in the inhibition of the reproduction of bacteria in a germicidal system, for the radical formation through synthesis of organic compounds like ozone and/or hydrogen-peroxide, or the activation of a catalyst in a UV/oxidation system. Since the quantum efficiency of the processes mentioned above is maximized if the emitted radiation matches the light-induced process, intense and a spectrally selective radiation in a narrow wavelength range is desired. A dielectric barrier discharge (DBD) lamp is able to emit a spectrally selective radiation in a narrow wavelength range, and may be used as a UV source in a reactor vessel. A DBD lamp can be realized when applying a high voltage across a gas gap, which is separated from metallic electrodes by at least one dielectric barrier. Dielectric barriers include, for instance, glass or quartz. Due to the nature of the DBD lamp to generate non-thermal plasmas at atmospheric gas pressure, this kind of lamp is effectively used to produce excited diatomic molecules (excimers) when using rare gases, or mixtures of rare gases and halogens as the discharge gas. The excimer emits radiation in the ultraviolet spectral range when it decays (in vacuum), which is used for various photo-initiated or photo-sensitized applications for water treatment. FIGS. 2A and 2B provide an example of a typical DBD lamp.
DBD
FIG. 2A is a side view of a coaxial DBD lamp. The lamp envelope 200 is a transparent vessel that is typically made of glass or quartz. In common arrangements, an inner electrode 210, which is connected to a high voltage source, is separated from an outer mesh electrode 240, which is grounded. A loop of the dielectric barrier 220 touches both the inner and outer electrodes and space 230 created by the loop is filled with the plasma gases.
FIG. 2B provides an end-on view of the same coaxial DBD lamp shown in FIG. 1A. In FIG. 2B, it can be seen more clearly that the inner electrode 210 and the outer electrode 240 are circular in shape, and that the plasma gases 230 are sealed between the two electrodes.
In current systems, the use of a mesh electrode is essential for the outer electrode since the openings in the mesh allow the generated UV radiation to exit the UV source. Using a standard mesh wire electrode, the mesh typically covers 50% of the lamp, which results in a reduction in efficiency of 50%, since 50% of the excimers that are emitted from the DBD lamp will strike the mesh, and hence, will not be emitted into the fluid.
Radiant Efficiencies
The UV radiant efficiencies of a DBD driven excimer (V)UV light source depend on the electron densities and the electron distribution function, and can be xe2x80x9ccontrolledxe2x80x9d mainly by the applied voltage frequency and shape, gas pressure, gas composition, and gas gap distance. For the most efficient excimers Xe2, XeCl, XeBr and KrCl, which emit narrow-banded or quasi-monochromatic UV light at 172 nm, 308 nm, 282 nm, and 222 nm, respectively, typical efficiencies are in the 8-15% range for sinusoidal high voltage and lamp arrangement (the efficiencies take into account the absorption of UV light by the metallic mesh electrode).
These efficiencies can double if steep-rising high voltage pulses can be generated, but a source that generates these steep-rising high voltage pulses is not readily available. Still, what makes this light source unique is that almost all of the radiation is emitted selectively. For photo-initiated or photo-sensitized processes, the emission can be considered quasi-monochromatic.
Many photo-physical and photo-chemical processes (e.g., UV curing and bonding, lacquer hardening, polymerization, material deposition, and UV oxidation) are initiated by a specific wavelength (ideally the excimer light source will emit close to those wavelengths). This light source can be far more effective than a high-powered light source when the emission suits the required wavelengths for a particular photo-physical or photo-chemical process, and this is desirable for certain applications like UV irradiation of water where specific wavelengths of light are needed.
Handicaps of Prior Art Systems
While the germicidal effect of UV radiation is very strong at 254 nm (which is the reason why UV disinfection systems for drinking water typically utilize the resonance radiation from low pressure mercury lamps at 254 nm), some UV disinfection and UV oxidation systems use intense, spectrally selective radiation at wavelengths other than 254 nm, which does not help in the disinfection of water. Unfortunately, with the exception of low-pressure mercury lamps, UV lasers, and dielectric barrier discharge (V)UV sources, no other intense, spectrally selective UV radiation sources are commercially available.
Commercially available intense UV sources are mainly medium and high pressure mercury lamps, Xe-arc, and flash lamps. All of these sources emit light in a broad spectral range. Therefore, if intense UV light other than the resonance radiation of mercury is desired in UV disinfection and UV oxidation systems, the only solution is to use a broad-band or continuous UV source with the knowledge that only some of the emission will fall in the required critical wavelength range. Since most of the light is not needed, there is a lot of wastage.
Industrial Applicability
Various high-powered (V)UV light sources are presently being applied for different fluid treatment processes (such as disinfection and UV oxidation) in large scales. All of the applied (V)UV light sourcesxe2x80x94with the exception of low-pressure mercury lampsxe2x80x94emit radiation into a broad spectral range, although it has proven that only certain wavelengths are responsible for the photochemical process. Most of the emitted radiation is therefore lost.
As a result, there is a demand for high-powered (V)UV light sources that can emit selective wavelengths responsible for the desired photochemical processes. DBD lamps have emerged as such a (V)UV light source. Conventional DBD lamps, however, suffer the disadvantage that the outer mesh electrode severely inhibits the amount of light emitted from the DBD lamps. In addition, current high powered DBD lamps must undergo forced cooling which further complicates their usage in conventional UV treatment systems as depicted in FIG. 1.
The present invention relates to a DBD lamp used in fluid treatment systems, where the irradiated fluid is used as a low voltage outer electrode instead of a metallic wire mesh. One or more embodiments of the present invention uses an electrically conductive fluid, for example water that is not de-ionized, as a source of the grounded electrode for the DBD lamp. In one embodiment, the fluid is in immediate contact with the outer surface of the DBD lamp. In another embodiment, the inner high voltage electrode remains in the center of the coaxial tube assembly and provides high voltage across the gas to generate excimer formation. In yet another embodiment, the fluid, which is sealed against the lamp envelope by two O-ring seals, is flown through the discharge reactor for UV processing.
The direct contact of the fluid with the DBD light source provides a strong cooling for the high voltage lamp. Additionally, since the fluid is used as the outer electrode instead of the prior art wire mesh electrode, the amount of radiation emitted increases as there is no absorption by the metallic mesh or the quartz sleeve.